1. Field of the Invention
The present invention relates to an optical wavelength conversion and an optical wavelength converter module for converting a fundamental generated by a light source to a second harmonic having a wavelength which is 1/2 of the wavelength of the fundamental, with an optical wavelength converter device.
2. Description of the Prior Art
Various attempts have heretofore been made for converting the wavelength of a laser beam into a shorter wavelength based on the generation of a second harmonic by a nonlinear optical material. One example of an optical wavelength converter device for effecting such laser wavelength conversion is a bulk crystal type converter device as disclosed, for example, in "Introduction to Optical Electronics" written by A Yariv and translated by Kunio Tada and Takeshi Kamiya (published by Maruzen K.K.), pages 200-204. A phase matching process for KTP which is a biaxial crystal is described in detail by Yao, et al in J. Appl. Phys. Vol. 55, page 65 (1984).
Conventional optical materials of nonlinear optical constants for use as the above bulk crystal type converter device include inorganic materials such as LiNbO.sub.3 and KTP and organic materials such as MNA(2-methyl-4-nitroaniline) disclosed in Japanese Laid-Open Patent Publication No. 60-250334, NPP(N-(4-nitrophenyl)-L-prolinol), NPAN(N-(4-nitrophenyl)-N-methylaminoacetonitrile), and the like disclosed in J. Opt. Soc. Am. B. Vol. 4, page 977 (1987). These organic nonlinear optical materials such as MNA, NPP, and the like have larger nonlinear optical constants than the inorganic nonlinear optical materials such as LiNbO.sub.3 and KTP, and hence are advantageous in that their wavelength conversion efficiency is high, they have a high dielectric breakdown threshold value, and are subject to less optical damage.
The optical wavelength converter device of the bulk crystal type relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, any material which does not exhibit birefringence or exhibits only small birefringence cannot be employed even if it has high nonlinearity.
A fiber type optical wavelength converter device has been proposed to solve the above problem. The optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of a nonlinear optical material surrounded by cladding. One example of such an optical fiber is shown in the bulletin Vol. 3, No. 2, of the Microoptics Research Group of a Gathering of the Applied Physics Society, pages 28-32. Recently, many efforts are directed to the study of a fiber type optical wavelength converter device since it can easily effect gain matching between a fundamental and a second harmonic.
For increasing the wavelength conversion efficiency of the fiber type optical wavelength converter device, it is preferable to employ an optical material of high nonlinear optical constants as a core. Known optical materials of high nonlinear optical constants include those referred to above.
Where a fiber type optical wavelength converter device is constructed of a nonlinear optical material of the type described above, however, the wavelength conversion efficiency of the optical wavelength converter device is not so increased since the crystal is not oriented in such a direction as to be able to utilize the maximum nonlinear optical constants of the material. The wavelength conversion efficiency of the optical wavelength converter device is longer as the length of the device is larger. The nonlinear optical materials referred to above are however not suitable for making long optical wavelength converter devices because it is difficult to obtain a uniform monocrystal from those nonlinear optical materials.
The absorption edges of the above nonlinear optical materials, e.g., MNA and NPP, are in the vicinity of 450 nm and 480 nm, respectively. Therefore, it is difficult to generate a second harmonic in a blue range, with its wavelength being near 400 nm, by employing, as a fundamental light source, a semiconductor laser that is widely used at present which has an excitation wavelength close to 800 nm. This is also the case with the optical wavelength converter device of the bulk crystal type described above. Moreover, KTP, LiNbO.sub.3, and the like which are inorganic materials have an absorption edge of 400 nm or below. While these materials can produce a second harmonic in a blue range, they are disadvantageous in that the performance index of wavelength conversion is lower than that of organic materials by one figure or more. Likewise, since their performance index is low when obtaining second harmonics in longer wavelength ranges such as of green, red, and the like, the efficiency of wavelength conversion is low.